Information systems for handling numerous document and data formats are moving towards becoming open systems where different devices are tied to one another to provide solutions to customers' needs. A key factor in such open systems is enabling an electronic document to be printed so that the customer does not perceive any difference between versions printed on different output devices. In order to achieve complete device-independence, efficient methods of accurately altering image resolution and performing enhancement are required to take advantage of the technology. Hence, raster conversion technology, where a bitmap created for a first output device is altered so as to be printable on a second output device, has become an important aspect of the open system technology.
The present invention utilizes statistically generated look-up tables to improve document appearance, and more fundamentally, to enable printing of a document by converting from an original resolution to an output resolution equal to that of the printing device. The resulting image signals may then be used to drive devices at the output resolution without impacting spatially sensitive features within the input image. The present invention may be utilized to control a scanning beam where the beam varies in intensity and duration according to the pulses used to control it. For example, a laser beam may be used in a printer for selectively exposing areas on a photoreceptor. The latent electrostatic image formed on the photoreceptor by the beam exposure attracts developing toner, in proportion to the latent image charge level, to develop the image. As another example, a cathode ray tube uses an electron beam to scan a phosphorous screen. The electron beam may be varied in intensity and duration to accurately display information on the phosphor screen. In both examples, a pulse forming circuit responsive to the multiple-bit per pixel image signal may be used to generate video pulses to control the intensity and operation time of the respective beams.
Previously, various methods and apparatus have been used to vary the position and width of pulses used to control laser or CRT beams, and to alter the resolution of bitmapped images. The following disclosures may be relevant:
U.S. Pat. No. 4,437,122 to Walsh et al. teaches an improved method of converting low resolution images into images of higher resolution for printing so as to simultaneously increase density and smooth character edges. In a CRT display or hardcopy output apparatus, the invention is accomplished by converting an original pixel into a higher resolution 3.times.3 enhanced representation. The status of each of the nine elements in the enhanced representation is determined as a result of an examination of the neighboring pixels of the original pixel.
U.S. Pat. No. 4,544,264 and U.S. Pat. No. 4,625,222, both issued to Bassetti et al. describe enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scan lines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. No. 4,544,922 to Watanabe et al. teaches a smoothing circuit for an orthogonal matrix display. The circuit adds or removes a "small dot" on the display from either the first or last third of a dot clock (DCK) period that is one-third the period in which a standard dot of the original pattern is displayed.
U.S. Pat. No. 4,690,909 to Bassetti discloses a method and apparatus for enhancing the apparent resolution of electrophotographic printers using a gray dot replacement technique to enhance low resolution output. More specifically, gray or halftone halos are produced along image edges to provide smoothing, while reducing the width of the image regions to avoid thickening thereof as a result of the halo.
U.S. Pat. No. 4,814,890 to Karo describes an image communicating system for transmitting or receiving image data. In the system, the pixel density of the transmitting image data is converted in accordance with the recording density on the reception side to enable printing by a high-speed laser printer without a change in the image size.
U.S. Pat. No. 4,841,375 to Nakajima et al. discloses an image resolution conversion apparatus that converts image data having a predetermined pixel density to a pixel density matching that of a printer so as to enable printing by the printer. The pixel density converter includes: a conversion-pixel position detector for detecting the position of a converted pixel; an original-pixel extractor for extracting a reference original pixel; a conversion-pixel density operation circuit for calculating the density of a conversion pixel; a threshold-value setter for dynamically setting a threshold value; a binary encoding circuit for digitizing the conversion-image density; an input interface for inputting image data; an output interface for outputting image data; and a control circuit for controlling the input/output (I/O) and the conversion operations.
U.S. Pat. No. 4,847,641 and U.S. Pat. No. 5,005,139 to Tung disclose print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 4,933,689 to Yoknis describes a method for enhancing a displayed image in a laser exposed dot matrix format to produce softened edge contours, using three pulses, a central pulse plus leading and trailing enhancement pulses which are separated therefrom. The purpose of the leading and trailing pulses is to create a blurred or grayed region at the leading and trailing edges of each associated character.
U.S. Pat. No. 5,029,108 to Lung teaches an edge enhancement method and apparatus for dot matrix devices wherein a group of gradient mask matrices are applied to a matrix of pixels surrounding a "to be adjusted pixel" so as to determine the existence of an edge and the direction of the brightness change. Once determined, the factors are used to generate a code used to modify the to be adjusted pixel in order to enhance the smoothness of a segment transition.
U.S. Pat. No. 5,134,495 to Frazier et al. discloses a laser based imaging system that employs a resolution transformation method. The method uses the selective activation in overlapping areas between rasters (scan lines). In one embodiment, a single interleaved pixel, between two scan lines, is formed by the sum of up to six laser pulses at pixel points on adjacent scan lines. In some cases the laser pulses are of insufficient intensity to produce a dot or mark at the point on the scan line where the center of the pulse is received.
U.S. Pat. No. 5,138,339 to Curry et al. teaches methods and means for increasing the precision with which optical printers that utilize high gamma recording media, such as xerographic printers, spatially position transitions in the images they print. The invention provides microaddressable display systems for rendering two-dimensional images on photosensitive media. The microaddressability results from the overscanning of intensity modulated spots that superimpose multiple discrete exposures on the recording medium, wherein the separation of the exposure centers is significantly less than the spatial diameter of the spots.
U.S. Pat. No. 5,142,374 to Tajika et al. teaches an image recording apparatus for recording a gradient image by combining dots of different densities. The different density dots are produced by generating a pair of binary signals, one for light ink and one for dark ink, from image data received. The discharge of each ink from a bubble jet is controlled in accordance with the signals.
U.S. Pat. No. 5,150,311 to Long et al. discloses a system for producing print-dot data suitable for driving a hardcopy printing device. More specifically, the print-dot data is selectively obtained from a conversion operation carried out by a matrix and dot generator combination that respectively generate subtractive color components and a pattern of high resolution print-dots therefrom.
U.S. Pat. No. 5,193,008 to Frazier et al. further describes the resolution enhancement apparatus as one that includes the ability to rasterize the image to be printed at twice the resolution of the printer. The printer then outputs the higher resolution image using an interleaving technique that generates developable dots between scan lines by energizing corresponding dots on adjacent scan lines at a level that will not be developed, but where the overlapping portion of the two corresponding dots will be developable.
U.S. Pat. No. 5,206,741 to Shimura et al. discloses an image processing apparatus for processing image data to be output by a printing unit. A conversion unit converts pixel image data within an image memory into data having a resolution equal to the output resolution of the print mechanism.
Donald L. Ort in Character Edge Smoothing for Matrix Printing, Xerox Disclosure Journal, Vol. 6, No. 1, January/February 1981, p. 19, describes a method for improving the quality of printed characters. The method described was offsetting of the location of the spots used to produce the character edges by one-half pixel, using either mechanical or electronic means, in order to smooth the edges.
A number of the previously described patents and publications are summarized in Torrey Pines Research, Behind Hewlett-Packard's Patent on Resolution Enhancement.TM. Technology, (Becky Colgan ed., BIS CAP International, 1990) pp. 1-60, including concepts associated with resolution enhancement.
James C. Stoffel et al. in A Survey of Electronic Techniques for Pictorial Image Reproduction, IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981, incorporated by reference for its teachings, discloses image processing algorithms that can be used to transform continuous tone and halftone pictorial image input into spatially encoded representations compatible with binary output processes. A set of image quality and processing complexity metrics are also defined so as to evaluate a number of image processing algorithms with respect to their ability to reproduce continuous tone or halftone pictorial input.
L. Steidel in Technology Overview: Resolution Enhancement Technologies for Laser Printers, LaserMaster Corp., discusses three currently available implementations for vertical resolution enhancement, Resolution Enhancement Technology, Paired Scan Line Scheme, and TurboRes. In all cases, the horizontal resolution of the laser scanner is increased by increasing the clock speed. On the other hand, the vertical resolution is enhanced by combining the weaker laser energy from a brief laser flash, which leaves only residual or fringe energy on the image drum at the periphery of an adjacent pixel on a second scan line.
Robert P. Loce et al. in Facilitation of Optimal Binary Morphological Filter Design via Structuring Element Libraries and Design Constraints, Optical Engineering, Vol. 31, No. 5, May 1992, pp. 1008-1025, incorporated herein by reference, describes three approaches to reducing the computational burden associated with digital morphological filter design. Although the resulting filter is suboptimal, imposition of the constraints in a suitable manner results in little loss of performance in return for design tractability.
Mathematical Morphology in Image Processing, pp. 43-90 (Edward R. Dougherty ed., Marcel Dekker 1992), hereby incorporated by reference, describes efficient design strategies for the optimal binary digital morphological filter. A suboptimal design methodology is investigated for binary filters in order to facilitate a computationally manageable design.
Robert P. Loce et al., in Optimal Morphological Restoration: The Morphological Filter Mean-Absolute-Error Theorem, Journal of Visual Communications and Image Representation, (Academic Press), Vol. 3, No. 4, December 1992, pp. 412-432, hereby incorporated by reference, teaches expressions for the mean-absolute restoration error of general morphological filters formed from erosion bases in terms of mean-absolute errors of single-erosion filters. In the binary setting, the expansion is a union of erosions, while in the gray-scale setting the expansion is a maxima of erosions. Expressing the mean-absolute-error theorem in a recursive form leads to a unified methodology for the design of optimal (suboptimal) morphological restoration filters. Applications to binary-image, gray-scale signal, and order-statistic restoration on images are included.
Edward R. Dougherty et al., in Optimal mean-absolute-error hit-or-miss filters: morphological representation and estimation of the binary conditional expectation, Optical Engineering, Vol. 32, No. 4, April 1993, pp. 815-827, incorporated herein by reference, discloses the use of a hit-or-miss operator as a building block for optimal binary restoration filters. Filter design methodologies are given for general-, maximum-, and minimum-noise environments and for iterative filters.
In accordance with the present invention there is provided, in a system suitable for depicting a bitmap image, a method for converting an input bitmap having a first resolution into an output bitmap image having a second, reproducible resolution, wherein the second resolution is a non-integer multiple of the first resolution, said method comprising the steps of: (a) creating a plurality of look-up tables, wherein each look-up table represents a unique phase relationship between the first and second resolution bitmaps and where each look-up table entry represents the resulting output to be generated by the look-up table in response to a specific pattern of input pixels within a pixel observation window; (b) determining, as a function of the position of the pixel observation window, the phase relationship for an output pixel in the output bitmap; and (c) generating, in response to the phase relationship and the pattern of input pixels within a pixel observation window, a video signal for the output pixel.
In accordance with another aspect of the present invention, there is provided a method for converting the resolution of a bitmap image having a first resolution, to enable production of a printed output on an electronic reprographic system capable of producing a second resolution image in response to a series of pulse-width, position-modulated signals, wherein the second resolution is a non-integer multiple of the first resolution, comprising the steps of: (a) selecting a target pixel location in the bitmap image; (b) observing a set of pixels within a pixel observation window superimposed on the bitmap image, relative to the target pixel location; (c) generating an index pointer as a function of a subset of the set of pixels; (d) determining, as a function of the position of the pixel observation window within the bitmap image, a phase of the pixel observation window; and (e) generating, as a function of the index pointer and the phase of the pixel observation window, an output pixel to be reproduced by the electronic reprographic system.
In accordance with yet another aspect of the present invention, there is provided a method for converting the resolution of a bitmap image having a first resolution (M), to enable production of a printed output on an electronic reprographic system capable of producing a second resolution (N) image in response to a series of pulse-width, position-modulated signals, wherein the second resolution is a non-integer multiple of the first resolution, comprising the steps of: (a) selecting a target pixel location in the bitmap image; (b) observing a set of pixels within a pixel observation window superimposed on the bitmap image, relative to the target pixel location; (c) generating, as a function of the states of the set of pixels, a third bitmap image having a common resolution K, wherein K is a common multiple of M and N; and (d) subsampling the third bitmap image so as to generate a series of second resolution pulse-width, position-modulated signals for output by the electronic reprographic system.
In accordance with a further aspect of the present invention, there is provided an electronic printing apparatus capable of printing a bitmap image having a first resolution by producing a series of pulse-width, position-modulated signals at a second resolution wherein the second resolution is a non-integer multiple of the first resolution, comprising: pixel selection means for selecting a target pixel location in the bitmap image; image memory for recording the binary states of a set of pixels in the bitmap image located within a pixel observation window positioned relative to the target pixel location; a pixel state register for storing an address generated as a function of the binary states of a subset of the pixels stored in said image memory; phase control logic for determining a phase of the pixel observation window; and translating means, responsive to the value stored in said pixel state register and the phase of the pixel observation window, for outputting a pulse attribute word that defines the characteristics of the pulse-width, position modulated signal used to print the target pixel so as to enhance the resulting printed output.